This invention relates to a method of manufacturing stationary electromagnetic cores and, more particularly, to manufacturing electromagnetic cores for relay switches and similar devices.
Soft magnetic iron (SUYB in the Japanese Industrial Standard) or low-carbon steel, are the materials commonly used in the manufacture of electromagnetic cores. In order to optimize the magnetic properties of the cores, materials containing 0.005% or less carbon, 0.05% or less manganese and 0.03% sulphur by weight (w) are generally used. In particular, soft magnetic iron (SUYB), a low-carbon soft iron containing about 0.005% (w) carbon and about 0.43% (w) impurities such as silicon, manganese, phosphorus and sulfur, is used.
Electromagnetic cores are manufactured by subjecting the material to hot rolling, cold-drawing the roll to a wire rod of a predetermined diameter, annealing the rod and pressing the rod to form a head and a boss on the core. The core is magnetically annealed to optimize its magnetic properties and finally subjected to surface treatment, such as copper and nickel plating.
In order to optimize the magnetic properties of the low-carbon soft iron, the magnetic annealing processes are repeated at least five times. This repetition results in a high manufacturing cost. The omission of magnetic annealing would necessitate stress relief during the annealing step of the cold drawing. Stress relief results in lowering the electromagnetic properties and coarsening of the ferrite grain size. Thus, stress relief lowers the ductility the metal will have in the pressing operation and decreases its adaptability to plating.
Although the use of low-carbon steel lowers the cost of materials and shortens the cold drying process, a decarburization step is required during the annealing process in order to optimize the magnetic properties.